Temperatures are forecast to hit 40 degrees in Sydney's west in the coming week, reaching a peak on Thursday.
Penrith is forecast to reach 37 degrees on Wednesday and 40 degrees on Thursday, while Parramatta can expect 35 degrees on Wednesday and 36 on Thursday.
Sydney's CBD will reach tops of 32.
Bureau of Meteorology forecaster Nick Neynens said Penrith's forecast was well above the December average.
"The average maximum temperature for December in Penrith is 29.4 degrees," he said.
"The highest ever recorded temperature in December [for Penrith] is 42.3 degrees, so it's quite close."
A north-easterly sea breeze will stop coastal regions from reaching such high temperatures, he said.
This afternoon's weather will be a "different story" Mr Neynens said, with scattered isolated storms expected throughout the state.
"There could be a good storm through Sydney today," he said.
"Isolated storms [are forecast] right through the Sydney metro area down the South Coast to Nowra, all up the North Coast and inland as well, because this trough has got a lot of instability."
Mr Neynens said Sydney could expect good weather over the weekend as temperatures are set to hit the mid-to-high 20s with partially cloudy conditions tomorrow and sunny skies on Sunday.
"Sunday, it's actually pretty good weather throughout the state," he said, with the chance of rain mostly in the north-east of NSW and the Blue Mountains.
Sydney can expect a top of 24 degrees tomorrow, Parramatta will reach 25 and Penrith can expect 28.
Sunday will be slightly warmer with tops of 26 degrees in Sydney, 28 in Parramatta and 31 in Penrith.
The high temperatures come after an unusually long warm spell in Sydney in November, when there were seven consecutive days above 25 degrees
The story Storms expected today before summer heat hits next week first appeared on The Sydney Morning Herald.